In a wind turbine the attachment of the root section of the rotor blades to the wind turbine's rotor hub is a very critical joint. Thereby, it has to be made sure that the root section is stable enough to transfer forces from the wind turbine blade into the hub. As an interface to the hub it is known to use metal parts which are inserted or attached in other ways to the very end of the root section. For instance, U.S. Pat. No. 4,915,590 teaches to use so-called sucker rods which are placed inside of the main body of a rotor blade and into which bolts can be screwed which connect an interface section of the hub with the rotor blade itself. Such sucker rods can for instance be glued into the main body. They have threads inside to receive the bolts coming from the hub.
It is also known to use a conical cylinder with threaded holes for a similar road attachment. The rotor blade is then adhered to the conical cylinder either by gluing or by placing a curing laminate directly in contact with the metal root. The conical cylinder may for instance be fabricated of aluminium.
Today's rotor blades of wind turbines are usually made of fibre reinforced plastic. For blades exceeding a length of 30 meters it is often a problem that the prevailing reinforcement fibres run in a longitudinal direction of the rotor blade. Only a few fibres are circumferential, which may lead to an ovalisation of the root end (also called root ring) once the blade is removed from the moulding tool where it has been produced. Once the blade is subjected to gravity forces without having a support the ovalisation occurs. This presents a problem when mounting the blade to the wind turbine's hub or when further mechanical machining of the root end takes place, for instance the drilling of holes.